Yeah, I'm really confused. Why on earth would you laminate adhesive to banner material, especially 18oz? You know they sell sell material made just for doing stuff that sticks on windows? You can print right on it, it's pretty amazing. I think it's called self adhesive vinyl.
It sounds like your customer is referring to a window decal as a banner and since none of y'all seem to know what you're doing, you ran with it rather than clarify what they wanted. That's my take on it at least. Kinda like when someone wants their name on their truck door but call it a wrap?